1. Field of the Invention
The present invention relates to an external resonator type of a variable-wavelength semiconductor laser light source for use in coherent light communication/measurement technique fields.
2. Description of the Related Art
In coherent light communication/measurement techniques, a semiconductor laser (referred to as the LD, hereinafter) light source of a single mode oscillation type which can change its wavelength in a wide range and have a narrow spectral line width and a good set-wavelength accuracy has been demanded.
In specifications of such a light source, there has been demanded in the market a light source which can have a variable width of 100 nm, a wavelength setting resolution of 0.001 nm and a wave reproducibility of 0.001 nm and also which can be operated with a constant light output.
Such LD light sources of the single mode oscillation type capable of changing the wavelength in a wide rage as mentioned above generally include an LD light source of an external resonator type which uses a diffraction grating.
The operation of oscillation wavelength of an external resonator type of LD light source using a diffraction grating will be explained briefly, as follows.
The oscillation wavelength of the LD light source is given by the following relational equations (1) and (2). The light from the LD light source oscillates with a resonating wavelength .lambda..sub.M which is in the vicinity of the Bragg wavelength .lambda..sub.b leading to a small mirror loss. EQU .lambda..sub.M =2.times.n.times.L/M (1) EQU .lambda..sub.b =2.times.d.times.sin(.theta.)/m (2)
where
.lambda..sub.m is the resonance wavelength in the external resonator, PA1 M is the longitudinal mode (integer) in the external resonator, PA1 L is the length of the external resonator, PA1 n is the refractive index of the external resonator, PA1 .lambda..sub.b is a Bragg wavelength, PA1 d is a spacing between grooves in the diffraction grating (grating constant), PA1 .theta. is an incident angle to the diffraction grating (Littrow mounting), and PA1 m is an order of reflected light of diffraction grating (usually, m=1)
Therefore, in an LD light source using a diffraction grating, the resonance wavelength .lambda..sub.M can be changed by changing the Bragg wavelength .lambda..sub.b to vary M. That is, the wavelength can be changed in a wide range by changing the incident angle .theta. to the diffraction grating with allowing mode hop. In order to change the wavelength successively without causing mode hop, the length of the external resonator L may be changed so that the longitudinal mode M is always constant corresponding to the incident angle .theta. to the diffraction grating. For example, a sine bar mechanism is adopted for this purpose. The principle thereof will be explained briefly with reference to FIG. 9, as follows.
In FIG. 9, an LD 1 and a diffraction grating 2 form a Fabry-Pe.GAMMA.rot resonator. The resonance wavelength .lambda..sub.M thereof in the equation (1) is expressed as follows. ##EQU1## where L.sub.A is the length of an arm (sine bar).
Accordingly, the difference between the wavelengths .lambda..sub.M and .lambda..sub.b is as follows. EQU .lambda..sub.M -.lambda..sub.b =2.times.sin(.theta.).times.{(L.sub.A /M)-(d/m)} (4)
Therefore, when the light source is designed to satisfy a condition (L.sub.A /M)=(d/m), the wavelength shift can be set at zero regardless of the rotational angle 2 of the diffraction grating 2.
Thus, the wavelength can be continuously changed without the mode hop.
FIG. 10 is an exemplary arrangement of a prior art external resonator type variable-wavelength LD light source.
First, an LD unit 4 includes, as shown in FIG. 11, a Fabry-Pe.GAMMA.rot type LD 1 coated on its one end face with an anti-reflection film (referred to as the AR coating, hereinafter) la, lenses 5 and 6, and an optical isolator 7. Further mounted to the LD unit 4 are a temperature detecting element 8 and a Peltier element 9, both of which are controlled with respect to temperature by a temperature adjustment circuit (referred to as the ATC circuit, hereinafter) 10.
In the illustrated example, light going from an end face side of the AR coating la of the LD 1 is changed by the lens 5 to parallel or collimated light. The lens 5 comprises preferably a non-spherical lens or a combined or composite lens having a small liquid-level aberration.
An LD end face 1bnot having the AR coating la applied thereon as well as the diffraction grating 2 as an external reflecting mirror form an external resonator.
The diffraction grating 2 is mounted to a rotary mechanism 11 and to one end of an arm (sine bar) 3, and the arm 3 abuts at the other end against an end face of a parallel moving mechanism 12.
The parallel moving mechanism 12, which has the LD unit 4 fixedly mounted thereto, is arranged to be moved in parallel with directions shown by arrows X. Provided in the vicinity of the parallel moving mechanism 12 is a switch 13 as a position detecting means.
A motor unit 14 as a driver unit includes a motor and an encoder, the motor being driven according to a pulse signal received from a control unit 15.
The aforementioned LD unit 4, diffraction grating 2, rotary mechanism 11, arm 3, parallel moving mechanism 12, switch 13 and motor unit 14 are mounted on an optical system base 16.
The LD 1 is driven by an LD driving current I.sub.LD received from an LD driver circuit 17.
A beam splitter 18 is an optical device which acts to pass therethrough part of output light P.sub.O emitted from a variable-wavelength LD light source unit 19 as output light P.sub.O1 and to reflect part of the remaining output light P.sub.O as output light P.sub.O2. Further, a PD unit 20 receives the reflected light P.sub.O2, converts it to a voltage value V(P.sub.O2), and outputs the voltage value to the LD driver circuit 17.
Since the voltage value V(P.sub.O2) received from the PD unit 20 is fed back to the LD driver circuit 17 to control the LD driving current I.sub.LD, constant light output operation can be realized. When the constant light output operation is not carried out, on the contrary, it is also possible to control the LD driving current I.sub.LD with use of a signal I.sub.SET received from the control unit 15.
Explanation will next be made as to how to set wavelength in FIG. 10.
First, when power is supplied to the respective sections of the system, a pulse signal generated from the control unit 15 so that the motor of the motor unit 14 is driven to move the parallel moving mechanism 12 in parallel and to rotate the rotary mechanism 11 through the arm 3.
When the parallel moving mechanism 12 reaches a certain position, the switch 13 is turned ON. The position at which the switch 13 is turned ON, is assumed to be an origin position. When the switch 13 is turned ON, the switch outputs an origin detecting signal S.sub.G. The control unit 15, when recognizing the origin detecting signal S.sub.G, stops the output of the pulse signal to stop the motor. At the same time, the controller outputs a reset signal R.sub.e to reset the encoder of the motor unit 14. The then oscillation wavelength is measured by a wavelength meter and a measured value .lambda..sub.O is assumed to be an origin wavelength .lambda..sub.O.
After this, the wavelength .lambda..sub.O is set at the origin position.
When the wavelength is varied, it is relatively changed with the respect to .lambda..sub.O as a reference.
That is, the control unit 15 calculates a difference between a set wavelength .lambda..sub.SET (referred to as the set wavelength, hereinafter) and the origin wavelength .lambda..sub.O, and outputs to the motor of the motor unit 14 a pulse for causing the parallel moving mechanism 12 to be moved to a position corresponding to the set wavelength .lambda..sub.SET. Further, the encoder of the motor unit 14 outputs always to the control unit 15 a signal S indicative of the movement amount of the parallel moving mechanism 12 moved by the motor, and the control unit 15 can recognize the current set wavelength based on the signal S.
In FIG. 10, the aforementioned sine bar mechanism is employed so that the parallel movement of the parallel moving mechanism 12 in the arrow X direction causes change of the rotational angle 2 of the diffraction grating 2 and at the same time, as seen from the above equation (3), adjustment of the external resonator length L in response to a change in 2 realizes continuous variable wavelength without mode hop.
In the prior art external resonator type variable-wavelength LD light source, however, the oscillation wavelength varies due to the following reasons and thus the wavelength reproducibility is deteriorated.
In this connection, an error .DELTA..lambda..sub.MA is expressed as follows . EQU .DELTA..lambda..sub.MA =.DELTA..lambda..sub.M (T.sub.O)+.DELTA..lambda..sub.M (I.sub.LD)+.DELTA..lambda..sub.M (S)(5)
where, .DELTA..lambda..sub.M (T.sub.O) is a change in the oscillation wavelength caused by thermal expansion involved by a temperature change in the entire light source unit, .DELTA..lambda..sub.M (I.sub.LD) is a change in the oscillation wavelength caused by a change in the LD driving current, and .DELTA..lambda..sub.M (S) is a shift of an actual oscillation wavelength from the set wavelength (encoder output value), caused by mechanical factors.
First one of the error causes is expressed by .DELTA..lambda..sub.M (T.sub.O). In the prior art variable-wavelength LD light source, the wavelength accuracy and wavelength reproducibility are guaranteed only at a constant temperature (e.g., 25.degree. C.), so that the thermal expansion and refractive index change caused by the temperature change of the entire light source unit cause change of "n" and "L" in the afore-mentioned equation (1), thus changing the oscillation wavelength .lambda..sub.M.
Assuming that a change in the external resonator length is denoted by .DELTA.(n.times.L), then .DELTA..lambda..sub.M (T.sub.O) is expressed, as follows, from the afore-mentioned equation (1). EQU .DELTA..lambda..sub.M (T.sub.O)=(.lambda..sub.M /(n.times.L).times..DELTA.(n.times.L) (6)
In this case, in the variable-wavelength LD light source unit 19 shown in FIG. 10, mechanical parts are made of mainly invar material having a low thermal expansion coefficient, but the motor of the motor unit 14, diffraction grating 2, lenses 5 and 6, LD 1, etc. are made of stainless steel, glass, quartz, etc. respectively.
In case that the thermal expansion coefficient of the external resonator including its refractive index change is about 5.times.10.sup.-6 and "n.times.L" in the above equation (1) is 30 mm when the variable-wavelength LD light source unit 19 is designed by using the above-described mechanical parts, "n.times.L" varies with a rate of 0.15 .mu.m/.degree.C.
When the light source is used at a temperature in the range of 25.+-.5.degree. C., .DELTA.(n.times.L).apprxeq.1.5 .mu.m. Hence, if .lambda..sub.M =1550 nm, then the wavelength varies by .DELTA..lambda..sub.M (T.sub.O)=0.07 nm in accordance with the afore-mentioned equation (6).
Second one of the error causes comes from .DELTA..lambda..sub.M (I.sub.LD) in the afore-mentioned equation (5), which results from a change in the LD driving current.
Assuming now that a rate of change of oscillation wavelength to the change of the LD driving current is denoted by d.lambda..sub.M /dI.sub.LD and a change in the LD driving current is denoted by .DELTA.I.sub.LD, then .DELTA..lambda..sub.M (I.sub.LD) is written as follows. EQU .DELTA..lambda..sub.M (I.sub.LD)=(d.lambda..sub.M /dI.sub.LD).times..DELTA.I.sub.LD (7)
As a specific value for the rate of change of the oscillation wavelength to the LD driving current change, for example, d.lambda..sub.M /dI.sub.LD was about 2 pm/mA as an actually measured value of a DFB single laser.
The afore-mentioned equation (1) is rewritten with use of parameters in FIG. 12, as follows. EQU .lambda..sub.M =2.times.(n.sub.O .times.L.sub.O +n.sub.LD .times.L.sub.LD)/M(8)
where n.sub.O is the refractive index (=1) of air, n.sub.LD is the refractive index of the LD, L.sub.O is "L-L.sub.LD " in FIG. 12, and L.sub.LD is the physical length of the LD.
In the case where d.lambda..sub.M /dI.sub.LD for the above single laser is considered as the external resonator type LD light source, if L.sub.LD =300 .mu.m, n.sub.LD =3.54, L=30 mm, n.sub.O =1 and .lambda..sub.M =1550 nm, then d.lambda..sub.M /dI.sub.LD .apprxeq.0.07 pm/mA in accordance with the above equations (1) and (8).
Assume now that the LD driving current is constant and the oscillation wavelength was changed by 100 nm. Then an optical output P.sub.0 is changed by about 1 mW from 4 mW as shown in FIG. 13 due to, e.g., wavelength.
In order to make the optical output constant, it is necessary to change the LD driving current as shown in FIG. 14. A relationship between P.sub.0 and I.sub.LD is as shown in FIG. 15, that is: EQU .DELTA.P.sub.0 /.DELTA.I.sub.LD .apprxeq.0.1 mW/mA (9)
Where .DELTA.P.sub.0 denotes an optical output change. Since .DELTA.P.sub.0 .apprxeq.=3 mW, .DELTA.I.sub.LD .apprxeq.30 mA.
Hence the wavelength is changed by .DELTA.I.sub.LD (I.sub.LD).apprxeq.2 pm in accordance with the above equation (7).
Third one of the error causes is .DELTA..lambda..sub.M (S) in the above equation (5), which is generated, in such an arrangement as shown in FIG. 10, by mechanical factors when the parallel moving mechanism 12 and rotary mechanism 1 are finely moved by the motor of the motor unit 14 to change the oscillation wavelength.
Theoretically, as shown by a dotted line in FIG. 16, the set wavelength .lambda..sub.SET corresponding to the output value of the encoder of the motor unit 14 and its actual oscillation wavelength vary proportionally in a 1:1 relationship; but they actually vary as shown by a solid line in FIG. 16.
The above-described 3 errors cause deterioration of the wavelength reproducibility. When the deterioration of the wavelength accuracy is taken into account, it becomes necessary, in addition to the above 3 errors, to take into consideration the error of .lambda..sub.O corresponding to the origin at the time of turning ON the power supply.
With regard to .lambda..sub.O, mechanical factors cause an random error in the external resonator length L in the equation (1).
Assuming now that a change in the external resonator length is denoted by .DELTA.L and an origin wavelength error is denoted by .DELTA..lambda..sub.O, then the following equation is satisfied in accordance with the equation (1). EQU .DELTA..lambda..sub.O =(.lambda..sub.O /L).times..DELTA.L (10)
When .lambda..sub.O =1550 nm, L=30 mm and the random error .DELTA.L=1 .mu.m, .DELTA..lambda..sub.O .apprxeq.0.05 nm in accordance with the equation (10).
In this case, an error of about 0.05 nm takes place for all the oscillation wavelengths .lambda..sub.M.
However, this error is not reproducible so that, if compensation is required, then it is necessary to re-measure the origin wavelength .lambda..sub.O each time the power supply is turned ON with use of a wavelength meter.
This may allow compensation of an error in the wavelength accuracy, but this also involves a large-scale light source with high costs, which is not practical.